The best way to do it is to buy an “angle cube” from Amazon or one of the other online dealers. This way you can get an exact angle of attack for each stone and adjust as necessary.
I have the iguaging cube from Amazon:
http://www.amazon.com/iGaging-Digital-Magnetic-Level-Protractor/dp/B002LL0BIC/ref=sr_1_1?ie=UTF8&qid=1320953566&sr=8-1
TedP